1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of thin film transistors (hereinafter, referred to as TFTs) and a method of manufacturing the semiconductor device. For example, the present invention relates to an electro-optical device typified by a liquid crystal display panel and an electronic equipment mounted with the electro-optical device as a component.
Note that the term semiconductor device in this specification indicates devices in general capable of functioning with the use of semiconductor characteristics, and electro-optical devices, semiconductor circuits and electronic equipment are all included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, a technology of constituting a thin film transistor (TFT) by using a semiconductor thin film (with a thickness of approximately several to several hundred of nm) formed on a substrate having an insulating surface has attracted attention. The thin film transistor is widely applied to an electronic device such as an IC or an electro-optical device, and needs to be developed promptly as, in particular, a switching element of an image display device.
An active matrix liquid crystal module is known as a typical example of the thin film transistors. Particularly, a TFT having a silicon film having a crystalline structure (typically, polysilicon film) as an active layer (hereafter, referred to as polysilicon TFT) has high filed-effect mobility compare to a TFT having a silicon film having a amorphous structure (typically, amorphous silicon film), and thus such TFTs are multiused recently.
Although there are various technologies of obtaining the silicon film having crystalline structure, especially, a technology given in Japanese Unexamined Patent Publication No. Hei. 8-78329 official report, in which the metallic elements (typically nickel) promoting crystallization to an amorphous silicon film are added alternatively, thereby performing a heat treatment to form the crystalline silicon film which spreads with an addition region as the starting point. Since the size of the crystal grain obtained thereof is very large compared with other technologies, and the field effect mobility is high, various circuits equipped with various functions can be formed thereby. For example, in case of using the technology of the above-mentioned official report, to a liquid crystal module carried in a liquid crystal display device, drive circuits for controlling pixel portions, such as pixel portions which perform an image display for every functional block, a shift register circuit based on a CMOS circuit, a level shifter circuit, a buffer circuit, and a sampling circuit, and the like can form on one substrate.
Moreover, the above-mentioned official report technology can lower approximately 50-100xc2x0 C. crystallization temperature of an amorphous silicon film by the action of metallic elements compared to a method without using metallic elements, thereby a glass substrate can be used without any problems occurring in process. Moreover, required time in the crystallization of the above-mentioned official report technology can be reduced to ⅕-{fraction (1/10)} compared to the method without using metallic elements, thereby the above-mentioned official report technology is also excellent in productivity.
A new further improvement is added to the technology of the above-mentioned official report, the manufacturing method of improving the film characteristic of a semiconductor film having a crystalline structure, and TFTs in which such a semiconductor film used as an active layer, excellent in the TFTs characteristics, such as the field effect mobility, are offered.
Considering the results of many experiments performed from a wide variety of fields in order to resolve the aforementioned various problems has lead to the present invention. When heat treatment is performed for crystallization, it is preferable to reduce the concentration of oxygen, which impedes crystallization, within a semiconductor film having an amorphous structure to which a metallic element is added for promoting crystallization, to a value as small as possible, specifically to less than 5xc3x971018/cm3. It was discovered that the above problems can be resolved, in particular field effect mobility can be increased, by performing the introduction of oxygen into the film after performing heat treatment.
The oxygen concentration within the film may be set from 5xc3x971018/cm3 to 1xc3x971021/cm3by irradiating laser light under an inert gas atmosphere, or in a vacuum, after oxidizing a surface of the semiconductor having a crystalline structure by using ozone water as a processing of introducing oxygen into the semiconductor film having a crystalline structure.
Alternatively, the oxygen concentration within the film may be set from 5xc3x971018/cm3to 1xc3x971021/cm3 by irradiating laser light under an atmosphere containing oxygen or water molecules as another process of introducing oxygen into the semiconductor film having a crystalline structure.
In addition, the oxygen concentration within the film may be set from 5xc3x971018/cm3 to 1xc3x971021/cm3 by irradiating laser light under an inert gas atmosphere, or in a vacuum, after performing oxidation under an atmosphere containing oxygen or water molecules by using an electric furnace or the like. Further, the oxygen concentration within the film may be set from 5xc3x971018/cm3 to 1xc3x971021/cm3 by irradiating laser light under an inert gas atmosphere, or in a vacuum, after adding oxygen by ion doping or ion implantation so that the oxygen concentration within the semiconductor film becomes 5xc3x971018/cm3 to 1xc3x971021/cm3. Furthermore, the semiconductor film is melted instantaneously from the surface, after which the melted semiconductor film is cooled and solidified from the substrate side because of thermal conduction to the substrate, for cases in which laser light is irradiated to the semiconductor film. Recrystallization takes place during the solidification process, and the semiconductor film becomes the one having a crystalline structure with a large grain size, but volumetric expansion develops due to the temporary melting, and unevenness referred to as ridges forms in the semiconductor surface. In particular, the surface on which the ridges form becomes an interface with a gate insulating film for top gate TFTs, and therefore the element characteristics vary greatly. In addition to the above processes, the oxide film on the semiconductor film surface is removed after laser light irradiation according to the present invention, and in addition, laser light is then irradiated under an inert gas atmosphere, or in a vacuum to level the surface of the semiconductor film having a crystalline structure.
Note that, differing from a technique for performing crystallization of the film having an amorphous structure by a first laser light and leveling by using a second laser light (JP 2001-60551 A), the present invention concerns irradiating the first laser light to the semiconductor film having a crystalline structure. Further, the present invention is the one in which a metallic element for promoting crystallization is added, a semiconductor film having a crystalline structure is formed, and levelness is additionally increased by the addition of the metallic element.
A first aspect of the present invention disclosed by this specification relates to a method of manufacturing a semiconductor device, including:
a first step of forming a semiconductor film having an amorphous structure on an insulating surface;
a second step of adding a metallic element to the semiconductor film having an amorphous structure;
a third step of heat-treating the semiconductor film having an amorphous structure to form a semiconductor film having a crystalline structure, and then removing an oxide film from the crystalline semiconductor film surface;
a fourth step of introducing oxygen into the semiconductor film having a crystalline structure to make an oxygen concentration within the film from 5xc3x971018/cm3 to 1xc3x971021/cm3;
a fifth step of removing an oxide film on the surface of the semiconductor film having a crystalline structure; and
a sixth step of irradiating laser light under an inert gas atmosphere or in a vacuum to level the surface of the semiconductor film having a crystalline structure.
Further, although the oxide film is formed on the surface when heat-treating the semiconductor film having an amorphous structure, the process of introducing oxygen may also be performed without removing the oxide film. A second aspect of the present invention relates to another method of manufacturing a semiconductor device, including:
a first step of forming a semiconductor film having an amorphous structure on an insulating surface;
a second step of adding a metallic element to the semiconductor film having an amorphous structure;
a third step of heat-treating the semiconductor film having an amorphous structure to form a semiconductor film having a crystalline structure;
a fourth step of introducing oxygen into the semiconductor film having a crystalline structure to make the oxygen concentration within the film from 5xc3x971018/cm3 to 1xc3x971021/cm3;
a fifth step of removing an oxide film on the surface of the semiconductor film having a crystalline structure; and
a sixth step of irradiating laser light under an inert gas atmosphere or in a vacuum to level the surface of the semiconductor film having a crystalline structure.
Furthermore, in the present invention, although the metallic element for promoting crystallization (typically Ni) is added onto the semiconductor film having an amorphous structure so as to cause crystallization, it is preferable that the metallic element for promoting crystallization be removed by a gettering technique or the like after crystallization. A third aspect of the present invention relates to another method of manufacturing a semiconductor device, including:
a first step of forming a semiconductor film having an amorphous structure on an insulating surface;
a second step of adding a metallic element to the semiconductor film having an amorphous structure;
a third step of heat-treating the semiconductor film having an amorphous structure to form a semiconductor film having a crystalline structure, and then removing an oxide film from the crystalline semiconductor film surface;
a fourth step of introducing oxygen into the semiconductor film having a crystalline structure to make the oxygen concentration within the film from 5xc3x971018/cm3 to 1xc3x971021/cm3;
a fifth step of removing an oxide film on the surface of the semiconductor film having a crystalline structure;
a sixth step of irradiating laser light under an inert gas atmosphere or in a vacuum to level the surface of the semiconductor film having a crystalline structure; and
a seventh step of gettering the metallic element to remove the metallic element from, or reduce the concentration of the metallic element within, the semiconductor film having a crystalline structure.
Further, in each of the aforementioned aspects of the invention, the energy density of the laser light used in performing the sixth step is set to 430 to 560 mJ/cm2, and the laser light irradiation performed by the fourth step uses laser light having an energy density that is lower by 30 to 60 mJ/cm2 than that of the laser light used by the sixth step (between 400 and 500 mJ/cm2).
Further, semiconductor films having the crystalline structure obtained by the above manufacturing method are included in the present invention. An aspect of a semiconductor device containing the semiconductor film having a crystalline structure of the present invention includes a TFT having:
a semiconductor layer having a channel formation region, a drain region, and a source region;
a gate insulating film; and
a gate electrode, in which:
a metallic element is contained within the semiconductor layer at a concentration of 1xc3x971016/cm3 to 5xc3x971018/cm3; and
average surface roughness (Ra value) of a surface of the semiconductor layer is equal to or less than 2 nm as obtained by AFM (atomic force microscopy).
Note that the metallic element in the above aspect is a metallic element for promoting crystallization of silicon, and is one element, or a plurality of elements, selected from the group consisting of Fe, Ni, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au.
Further, extremely unique data on a state of the film surface is also obtained for the semiconductor film having a crystalline structure of the present invention, at the same time as data is obtained on superior levelness, by using AFM (atomic force microscopy). For cases in which a metallic element which promotes crystallization is not used, a tortoise shell pattern is formed surrounded by ridges (portions in which microscopic convex portions extend continuously). An irregular mesh pattern in which several regions exist, divided by ridges extending in many directions as shown in FIG. 3, can be observed, however, as the surface state of the semiconductor film having a crystalline structure of the present invention in which crystallization is performed using a metallic element which promotes crystallization. Regions sandwiched by the ridges (level portions and concave portions) correspond well to an aggregation of crystal grains having the same crystal orientation (also referred to as domains).
The semiconductor film of the present invention has an irregular mesh pattern in the semiconductor film surface, as shown in FIG. 3. Ridges having convex portions extending out in a ridge shape diverge in many directions, and there is at least one pathway not obstructed by the ridges between two arbitrary points in a region containing level portions and concave portions sandwiched irregularly by the ridges. Note that the ridges are formed by performing laser light irradiation a plurality of times.
Further, the ridges having convex portions that extend out in a ridge shape with forming an irregular mesh pattern are formed in locations that nearly correspond to individual domain boundaries. The fact that the individual domain boundaries and the ridges nearly correspond can be verified by a method referred to as unique grain mapping (in which an electron beam is scanned over a sample, and from the crystal orientations found at each point, regions are classified in which crystal orientations have an angular shift less than 15xc2x0 between two adjacent points at the respective measurement points). Here, SEM observation photograph and electron backscatter diffraction pattern (EBSP) are used in the analysis in the same region. That is, in addition to the fact that there is at least one pathway not obstructed by the ridges between two arbitrary points in a region containing level portions and concave portions sandwiched irregularly by the ridges, there is a pathway between two arbitrary points in a region sandwiched by domain boundaries in which the shift between adjacent points in crystal orientations is less than 15xc2x0. This can be expected to be a factor in obtaining a semiconductor film having superior electrical characteristics, in particular, superior field effect mobility.
Further, the above surface state and crystal orientation characteristics are characteristic of the present invention and cannot be obtained by another method. The characteristic can first be seen after adding a metallic element for promoting crystallization (typically nickel), crystallizing by performing heat treatment, and in addition, removing an oxide film on the semiconductor film surface after performing irradiation of a first laser light, and leveling the surface of the semiconductor film having a crystalline structure by irradiating laser light under an inert gas atmosphere or in a vacuum.
Also, in the aforementioned semiconductor film, a metallic element is contained therein at a concentration of 1xc3x971016/cm3 to 5xc3x971018/cm3. Furthermore, the semiconductor film is level, having an average surface roughness (Ra value) equal to or less than 2 nm.
Further, a semiconductor device having superior electrical characteristics can be obtained by using the semiconductor film as a portion of the semiconductor device, for example as an active layer of a TFT.
An aspect of a semiconductor device of the present invention includes a TFT having:
a semiconductor layer having a channel formation region, a drain region, and a source region;
a gate insulating film; and
a gate electrode, in which:
a surface of the semiconductor layer has an irregular mesh pattern;
ridges having convex portions that extend out in a ridge shape diverge in many directions; and
at least one pathway that is not obstructed by the ridges is provided between two arbitrary points in a region containing a level portion and a concave portion sandwiched irregularly by the ridges. A metallic element is contained within the aforementioned semiconductor layer at a concentration of 1xc3x971016/cm3 to 5xc3x971018/cm3. Furthermore, the semiconductor layer is level, having an average surface roughness (Ra value) equal to or less than 2 nm.
The crystalline structure for cases in which crystallization is performed by a conventional solid phase growth method becomes a twin structure, and a semiconductor film contains a large number of twin defects within the crystal grains. In contrast, a plurality of rod shape crystal grain aggregates (domains) are formed in a semiconductor film obtained by the present invention, and all of the crystal grains of a certain crystal grain aggregate (domain) can be considered to have the same crystal orientation. The size of the crystal grain aggregate (domain) is equal to or greater than approximately 1 xcexcm, with the large ones having a size of several tens of micrometers.
Further, the number of defects contained in the grain boundaries within one domain (unbonded hands of silicon) is extremely small, and the electrical barrier is small, compared to the grain boundaries obtained by the conventional solid phase growth methods or the like. That is, the interior of one domain is approximately close to a single crystal, and it is thought that the film characteristics will become more superior, the larger the domain size becomes.
The term adjacent crystal aggregates (domains) refers to aggregates having different orientations with a boundary (portion in which a microscopic convex portion extends continuously) between the aggregates. Similarly, the surface state can also be observed by using SEM observation.
Note that FIG. 3 is a diagram showing AFM observation after performing crystallization by using heat treatment, irradiating laser light under an atmosphere containing oxygen as a process of introducing oxygen into the film, removing an oxide film on the surface, and then performing leveling by irradiating laser light under a nitrogen atmosphere. On the other hand, FIG. 2 is a diagram showing AFM observation after performing crystallization by using heat treatment, and irradiating laser light under an atmosphere containing oxygen as a process of introducing oxygen into the film, but it is difficult to see domain boundaries. As described above, individual domain boundaries can be confirmed by AFM and SEM by removing an oxide film on the surface, and then irradiating laser light under an inert atmosphere or in a vacuum. Note that, except for making the film surface flat and allowing individual domains to be clearly visualized, the irradiation of laser light under an inert atmosphere or in a vacuum imparts almost no changes to the semiconductor film or to the crystalline state. That is, the size of the domains obtained by the present invention is determined by the processes performed before irradiating the laser light under an inert atmosphere or in a vacuum (such as processes for forming a semiconductor film having an amorphous structure, heat treatment for crystallization, and processes for introducing oxygen).